Organic light emitting diode display including capping layer having optical thickness for improving optics

ABSTRACT

An OLED display including first, second, and third color pixels each including a first electrode, an organic emission layer, a second electrode, and a capping layer disposed on the disposed on the substrate, in which the first color pixel is configured to emit green light, and the second and third color pixels are each configured to emit a color of light other than green, the organic emission layer of the first color pixel includes a first emission layer and a second emission layer each being configured to emit light, the organic emission layer of the second color pixel or in the third color pixel includes a third emission layer configured to emit light, the second emission layer and the third emission layer include both a host and a dopant, and the first emission layer includes the host, and does not include any dopants therein.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.15/097,871, filed on Apr. 13, 2016, and claims priority from and thebenefit of Korean Patent Application No. 10-2015-0138975, filed in onOct. 2, 2015, which are hereby incorporated by reference for allpurposes as if fully set forth herein.

BACKGROUND Field

Exemplary embodiments relate to an organic light emitting diode (OLED)display, and, more particularly, to an OLED display with increasedluminous efficiency and improved viewing angle.

Discussion of the Background

An organic light emitting diode (OLED) display may include twoelectrodes and an organic emission layer interposed therebetween. Theelectrons injected from one electrode and holes injected from the otherelectrode may be combined in the organic emission layer to generateexcitons. Light may be emitted as the excitons release energy, when theexcitons change from an excited state to a ground state.

An OLED display includes pixels including an OLED, as a self-emissiveelement, transistors for driving the OLED, and at least one capacitorformed in each pixel. The transistors may generally include a switchingtransistor and a driving transistor.

A capping layer may be formed on the electrodes to protect theelectrodes of the OLED display. Accordingly, luminous efficiency oflight emitted from the organic emission layer may be affected by thecapping layer. However, as luminous efficiency increases, strongresonance may occur, which may generate color shift associated with aviewing angle. That is, a difference in color may appear between frontand side views of a screen.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the inventive concept,and, therefore, it may contain information that does not form the priorart that is already known in this country to a person of ordinary skillin the art.

SUMMARY

Exemplary embodiments provide an organic light emitting diode (OLED)display that may improve luminous efficiency thereof.

Exemplary embodiments also provide an OLED display that may improve aviewing angle by reducing color shift associated with a viewing angle.

Additional aspects will be set forth in the detailed description whichfollows, and, in part, will be apparent from the disclosure, or may belearned by practice of the inventive concept.

According to an exemplary embodiment of the present invention, anorganic light emitting diode (OLED) display includes a substrate, afirst electrode disposed on the substrate, an organic emission layerdisposed on the first electrode, a second electrode disposed on theorganic emission layer, and a capping layer disposed on the secondelectrode, in which an optical thickness of the capping layer is in arange of about 1100 Å to about 1400 Å.

According to an exemplary embodiment of the present invention, anorganic light emitting diode (OLED) display includes a substrate, afirst electrode disposed on the substrate, an organic emission layerdisposed on the first electrode, a second electrode disposed on theorganic emission layer, and a capping layer disposed on the secondelectrode, the capping layer including a first capping layer and asecond capping layer having different refractive indices from eachother, in which an optical thickness of the first capping layer is in arange of about 60 Å to 210 Å, and an optical thickness of the secondcapping layer is in a range of about 820 Å to about 1030 Å.

According to an exemplary embodiment of the present invention, anorganic light emitting diode (OLED) display includes a first colorpixel, a second color pixel, and a third color pixel on a substrate,each of the first, second, and third color pixels including a firstelectrode disposed on the substrate, an organic emission layer disposedon the first electrode, a second electrode disposed on the organicemission layer, and a capping layer disposed on the second electrode, inwhich the first color pixel is configured to emit green light, and thesecond and third color pixels are each configured to emit a color oflight other than green, the organic emission layer of the first colorpixel includes a first emission layer and a second emission layer eachbeing configured to emit light, the organic emission layer of the secondcolor pixel or the third color pixel includes a third emission layerconfigured to emit light, the second emission layer and the thirdemission layer include both a host and a dopant, and the first emissionlayer includes the host, and does not include any dopants therein.

According to exemplary embodiments of the present invention, an opticalthickness of the capping layer may be controlled to improve the luminousefficiency and the viewing angle. In addition, the luminous efficiencyand the viewing angle may be improved by including the host and thedopant in a first portion of regions of the emission layer and only thehost in a second portion of regions thereof.

The foregoing general description and the following detailed descriptionare exemplary and explanatory and are intended to provide furtherexplanation of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the inventive concept, and are incorporated in andconstitute a part of this specification, illustrate exemplaryembodiments of the inventive concept, and, together with thedescription, serve to explain principles of the inventive concept.

FIG. 1 is an equivalent circuit diagram of one pixel of an organic lightemitting diode (OLED) display according to an exemplary embodiment ofthe present invention.

FIG. 2 is a cross-sectional view of an OLED display according to anexemplary embodiment of the present invention.

FIG. 3 is an enlarged cross-sectional view of an OLED of an OLED displayaccording to an exemplary embodiment of the present invention.

FIG. 4 is a graph showing luminous efficiency of white light associatedwith a thickness of a capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 5 is a graph showing luminous efficiency of red light associatedwith a thickness of a capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 6 is a graph showing luminous efficiency of green light associatedwith a thickness of a capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 7 is a graph showing luminous efficiency of blue light associatedwith a thickness of a capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 8 is a graph showing an amount of color shift for white associatedwith a thickness of a capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 9 is a graph showing an amount of color shift for red associatedwith a thickness of a capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 10 is a graph showing an amount of color shift for green associatedwith a thickness of a capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 11 is a graph showing an amount of color shift for blue associatedwith a thickness of a capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 12 is a cross-sectional view of an organic emission layer of anOLED display according to an exemplary embodiment of the presentinvention.

FIG. 13 is a cross-sectional view of an organic emission layer of anOLED display according to an exemplary embodiment of the presentinvention.

FIGS. 14 and 15 are cross-sectional views of an organic emission layerof an OLED display according to exemplary embodiments of the presentinvention.

FIG. 16 is a graph showing an amount of color shift associated with astructure of an emission layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 17 is a graph showing a driving voltage associated with thestructure of an emission layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 18 is a graph showing luminous efficiency associated with thestructure of an emission layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 19 is a cross-sectional view of an OLED of an OLED displayaccording to an exemplary embodiment of the present invention.

FIG. 20 is a graph showing luminous efficiency associated with athickness of a first capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 21 is a graph showing luminous efficiency associated with athickness of a second capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 22 is a graph showing an amount of color shift associated with thethickness of a first capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 23 is a graph showing an amount of color shift associated with thethickness of a second capping layer of an OLED display according to anexemplary embodiment of the present invention.

FIG. 24 is a cross-sectional view of an organic emission layer of anOLED display according to an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of various exemplary embodiments. It is apparent, however,that various exemplary embodiments may be practiced without thesespecific details or with one or more equivalent arrangements. In otherinstances, well-known structures and devices are shown in block diagramform in order to avoid unnecessarily obscuring various exemplaryembodiments.

In the accompanying figures, the size and relative sizes of layers,films, panels, regions, etc., may be exaggerated for clarity anddescriptive purposes. Also, like reference numerals denote likeelements.

When an element or layer is referred to as being “on,” “connected to,”or “coupled to” another element or layer, it may be directly on,connected to, or coupled to the other element or layer or interveningelements or layers may be present. When, however, an element or layer isreferred to as being “directly on,” “directly connected to,” or“directly coupled to” another element or layer, there are no interveningelements or layers present. For the purposes of this disclosure, “atleast one of X, Y, and Z” and “at least one selected from the groupconsisting of X, Y, and Z” may be construed as X only, Y only, Z only,or any combination of two or more of X, Y, and Z, such as, for instance,XYZ, XYY, YZ, and ZZ. Like numbers refer to like elements throughout. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

Although the terms first, second, etc. may be used herein to describevarious elements, components, regions, layers, and/or sections, theseelements, components, regions, layers, and/or sections should not belimited by these terms. These terms are used to distinguish one element,component, region, layer, and/or section from another element,component, region, layer, and/or section. Thus, a first element,component, region, layer, and/or section discussed below could be termeda second element, component, region, layer, and/or section withoutdeparting from the teachings of the present disclosure.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, may be used herein for descriptive purposes, and,thereby, to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the drawings. Spatiallyrelative terms are intended to encompass different orientations of anapparatus in use, operation, and/or manufacture in addition to theorientation depicted in the drawings. For example, if the apparatus inthe drawings is turned over, elements described as “below” or “beneath”other elements or features would then be oriented “above” the otherelements or features. Thus, the exemplary term “below” can encompassboth an orientation of above and below. Furthermore, the apparatus maybe otherwise oriented (e.g., rotated 90 degrees or at otherorientations), and, as such, the spatially relative descriptors usedherein interpreted accordingly.

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting. As used herein, thesingular forms, “a,” “an,” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. Moreover,the terms “comprises,” “comprising,” “includes,” and/or “including,”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, components, and/orgroups thereof, but do not preclude the presence or addition of one ormore other features, integers, steps, operations, elements, components,and/or groups thereof.

Various exemplary embodiments are described herein with reference tosectional illustrations that are schematic illustrations of idealizedexemplary embodiments and/or intermediate structures. As such,variations from the shapes of the illustrations as a result, forexample, of manufacturing techniques and/or tolerances, are to beexpected. Thus, exemplary embodiments disclosed herein should not beconstrued as limited to the particular illustrated shapes of regions,but are to include deviations in shapes that result from, for instance,manufacturing. For example, an implanted region illustrated as arectangle will, typically, have rounded or curved features and/or agradient of implant concentration at its edges rather than a binarychange from implanted to non-implanted region. Likewise, a buried regionformed by implantation may result in some implantation in the regionbetween the buried region and the surface through which the implantationtakes place. Thus, the regions illustrated in the drawings are schematicin nature and their shapes are not intended to illustrate the actualshape of a region of a device and are not intended to be limiting.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure is a part. Terms,such as those defined in commonly used dictionaries, should beinterpreted as having a meaning that is consistent with their meaning inthe context of the relevant art and will not be interpreted in anidealized or overly formal sense, unless expressly so defined herein.

Hereinafter, one pixel of an organic light emitting diode (OLED) displayaccording to an exemplary embodiment of the present invention will bedescribed.

FIG. 1 is an equivalent circuit diagram of one pixel of an OLED displayaccording to an exemplary embodiment of the present invention.

Referring to FIG. 1, the OLED display according to the present exemplaryembodiment includes signal lines 121, 171, and 172, and a pixel PXconnected thereto. Although not illustrated, pixels PX may be arrangedin a matrix form that includes pixel rows and pixel columns. In thiscase, the pixels PX are respectively connected to the signal lines 121,171, and 172.

The signal lines 121, 171, and 172 include gate lines 121 fortransmitting a gate signal (also referred to as a “scan signal”), datalines 171 for transmitting a data signal, and driving voltage lines 172for transmitting a driving voltage Vdd. Only one of the gate lines 121,one of the data lines 171, and one of the driving voltage lines 172 areillustrated in FIG. 1, but the gate line 121, data line 171, and drivingvoltage line 172 may be formed in plural. The gate lines 121substantially extend in a row direction and are substantially parallelto each other. The data lines 171 and the driving voltage lines 172extend in a column direction to cross the gate lines 121 and aresubstantially parallel to each other.

The OLED display according to the present exemplary embodiment includesa switching thin-film transistor Qs, a driving thin-film transistor Qd,a storage capacitor Cst, and an organic light emitting diode (OLED).

The switching thin-film transistor Qs includes a control terminal, aninput terminal, and an output terminal. The control terminal of theswitching thin-film transistor Qs is connected to the gate line 121, theinput terminal thereof is connected to the data line 171, and the outputterminal thereof is connected to the driving thin-film transistor Qd.The switching thin-film transistor Qs transmits the data signal appliedto the data line 171 to the driving thin-film transistor Qd, in responseto the gate signal applied to the gate line 121.

The driving thin-film transistor Qd has a control terminal, an inputterminal, and an output terminal. The control terminal of the drivingthin-film transistor Qd is connected to the switching thin-filmtransistor Qs, the input terminal thereof is connected to the drivingvoltage line 172, and the output terminal thereof is connected to theOLED. The driving thin-film transistor Qd outputs an output current ILD,an amount of which varies depending on a voltage applied between thecontrol terminal and the output terminal of the driving thin-filmtransistor Qd.

The storage capacitor Cst is connected between the control terminal andinput terminal of the driving thin-film transistor Qd. The storagecapacitor Cst is charged by the data signal applied to the controlterminal of the driving thin-film transistor Qd, and maintains the datasignal even after the switching thin-film transistor Qs is turned off.

The OLED has an anode connected to the output terminal of the drivingthin-film transistor Qd, and a cathode connected to a common voltageVss. The OLED displays an image by emitting light, the intensity ofwhich varies depending on the output current I_(LD) of the drivingthin-film transistor Qd. The connection relationship between theswitching thin-film transistor Qs, the driving thin-film transistor Qd,the storage capacitor Cst, and the OLED may vary widely. In addition,additional thin-film transistors may be included in one pixel PX.

Hereinafter, the OLED display according to an exemplary embodiment ofthe present invention will be described in detail.

FIG. 2 is a cross-sectional view of the OLED display according to anexemplary embodiment of the present invention.

Referring to FIG. 2, the OLED display according to the present exemplaryembodiment includes a substrate 110, a driving thin-film transistor Qdformed on the substrate 110, a pixel electrode 191 connected to thedriving thin-film transistor Qd, an organic emission layer 370 formed onthe pixel electrode 191, and a common electrode 270 formed on theorganic emission layer 370.

The substrate 110 is an insulating substrate that is formed of glass,quartz, ceramic, plastic, etc. The substrate 110 may alternatively be ametallic substrate that is formed of stainless steel or the like.

A buffer layer 120 may be disposed on the substrate 110. The bufferlayer 120 may be formed as a single layer of silicon nitride (SiN_(x)),or a dual layer of silicon nitride (SiN_(x)) and silicon oxide(SiO_(x)). The buffer layer 120 may planarize a surface and preventpermeation of unnecessary materials, such as impurities or moisture. Thebuffer layer 120 may be omitted.

A driving semiconductor 130 is disposed on the buffer layer 120. Thedriving semiconductor 130 may include a polysilicon semiconductormaterial or an oxide semiconductor material. The driving semiconductor130 includes a channel region 131 not doped with impurities, and contactdoping regions 132 and 133 doped with impurities, which are disposed atopposite sides of the channel region 131. The contact doping regions 132and 133 include a source region 132 and a drain region 133. In thiscase, the doped impurities may vary depending on a type of the thin-filmtransistor.

A gate insulating layer 140 is disposed on the driving semiconductor130. The gate insulating layer 140 may include an inorganic insulatingmaterial, such as silicon nitride (SiN_(x)) or silicon oxide (SiO_(x)).

A gate wire including the driving gate electrode 125 is disposed on thegate insulating layer 140. In this case, the driving gate electrode 125overlaps at least a portion of the driving semiconductor 130, inparticular, the channel region 131 thereof.

An interlayer insulating layer 160 is disposed on the driving gateelectrode 125 and the gate insulating layer 140. The interlayerinsulating layer 160 may include an inorganic insulating material or anorganic insulating material.

Contact holes 162 and 164 at least partially exposing the drivingsemiconductor 130 are formed in the gate insulating layer 140 and in theinterlayer insulating layer 160. Particularly, the contact holes 162 and164 expose the contact doping regions 132 and 133 of the drivingsemiconductor 130.

A data wire including a driving source electrode 173 and a driving drainelectrode 175 are formed on the interlayer insulating layer 160. Thedriving source electrode 173 and the driving drain electrode 175 arerespectively connected to the source and drain regions 132 and 133 ofthe driving semiconductor 130 via the contact holes 162 and 164.

In this manner, the driving semiconductor 130, the driving gateelectrode 125, the driving source electrode 173, and the driving drainelectrode 175 form the driving thin-film transistor Qd. It is notedthat, however, the structure of the driving thin-film transistor Qd maybe varied. The switching thin-film transistor Qs may include a switchingsemiconductor, a switching gate electrode, a switching source electrode,and a switching drain electrode.

A passivation layer 180 covering the data wire is disposed on theinterlayer insulating layer 160. The passivation layer 180 may remove astep and planarize a surface thereof, thereby increasing luminousefficiency of the OLED to be formed thereon. A contact hole 182 at leastpartially exposing the driving drain electrode 175 is formed in thepassivation layer 180.

The passivation layer 180 may include a polyacrylate resin, an epoxyresin, a phenolic resin, a polyamide resin, a polyimide resin, anunsaturated polyester resin, a polyphenylene resin, a polyphenylenesulfide resin, and benzocyclobutene (BCB). It is noted that, however,either one of the passivation layer 180 and the interlayer insulatinglayer 160 may be omitted.

The pixel electrode 191 is formed on the passivation layer 180. Thepixel electrode 191 may include a transparent conductive material, suchas indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO),indium oxide (In₂O₃), etc., or a reflective metal such as lithium (Li),calcium (Ca), lithium fluoride/calcium (LiF/Al), lithiumfluoride/aluminum (LiF/Al), aluminum (Ag), silver (Ag), magnesium (Mg),gold (Au), etc.

The pixel electrode 191 is electrically connected to the driving drainelectrode 175 of the driving thin-film transistor Qd via the contacthole 182 formed in the passivation layer 180, and may be the anode ofthe OLED. Although not illustrated, the pixel electrode 191 may includefirst and second transparent electrodes including a transparentconductive material, and a transflective layer disposed between thefirst and second transparent electrodes, to form a microcavity alongwith the common electrode 270.

A pixel defining layer 350 is formed on the passivation layer 180 and anedge portion of the pixel electrode 191. The pixel defining layer 350includes a pixel opening 351 that exposes the pixel electrode 191. Thepixel defining layer 350 may include a polyacrylate resin, a polyimideresin, a silica-based inorganic material, etc.

An organic emission layer 370 is formed in the pixel opening 351 of thepixel defining layer 350. The organic emission layer 370 may include atleast one of an emission layer, a hole injection layer (HIL), a holetransporting layer (HTL), an electron transporting layer (ETL), and anelectron injection layer (EIL). A structure of the organic emissionlayer 370 will be described in detail with reference to FIG. 3.

The organic emission layer 370 may include a red organic emission layerfor emitting red light, a green organic emission layer for emittinggreen light, and a blue organic emission layer for emitting blue light.The red organic emission layer, the green organic emission layer, andthe blue organic emission layer are respectively formed on red, green,and blue pixels, to implement a color image.

Alternatively, in the organic emission layer 370, a color image may beimplemented by disposing all of the red, green, and blue organic lightemission layers on the red pixel, the green pixel, and the blue pixel,and then forming red, green, and blue color filters for each pixel. Asanother example, a color image may be implemented by forming a whiteorganic emission layer emitting white light on all of the red, green,and blue pixels, and respectively forming red, green, and blue colorfilters for each pixel. When the color image is implemented by using thewhite organic emission layer and the color filter, a deposition mask fordepositing the red, green, and blue organic emission layers on eachpixel, that is, the red, green, and blue pixels, may not be utilized.

The white organic emission layer described above may be formed as asingle organic emission layer, and may further include a structure foremitting white light by disposing multiple organic emission layers. Forexample, a structure for emitting white light by combining at least oneyellow organic emission layer with at least one blue organic emissionlayer, at least one cyan organic emission layer with at least one redorganic emission layer, and at least one magenta organic emission layerwith at least one green organic emission layer may be included.

The common electrode 270 is formed on the pixel defining layer 350 andon the organic emission layer 370. The common electrode 270 may includea transparent conductive material, such as indium tin oxide (ITO),indium zinc oxide (IZO), zinc oxide (ZnO), indium oxide (In₂O₃), etc.,or a reflective metal such as lithium (Li), calcium (Ca), lithiumfluoride/calcium (LiF/Al), lithium fluoride/aluminum (LiF/Al), aluminum(Ag), silver (Ag), magnesium (Mg), gold (Au), etc. The common electrode270 may be the cathode of the OLED. The pixel electrode 191, the organicemission layer 370, and the common electrode 270 form the OLED.

A capping layer 500 is disposed on the common electrode 270. The cappinglayer 500 is formed in a top emission type of OLED, to prevent incidentlight on the common electrode 270 from being totally reflected, and,thus, experiencing light loss. The capping layer 500 may include anorganic material or an inorganic material. For example, the cappinglayer 500 may include a triamine derivative, an arylenediaminederivative, CBP, tris(8-hydroxyquinoline) aluminum (Alq₃), etc.

An optical thickness of the capping layer 500 may be about 1100 Å ormore and about 1400 Å or less. As used herein, an optical thickness maybe defined by the physical thickness multiplied by the refractive indexthereof. A refractive index of the capping layer 500 may be about 1.6 ormore and about 2.6 or less. For example, when the refractive index ofthe capping layer 500 is about 1.87, a physical thickness of the cappinglayer 500 may be about 600 Å or more and about 730 Å or less. That is,the optical thickness of the capping layer 500 is about 600 Å*1.87 ormore and about 730 Å*1.87 or less. When the refractive index of thecapping layer 500 is about 1.6, the physical thickness of the cappinglayer 500 may be about 700 Å or more and about 860 Å or less. When therefractive index of the capping layer 500 is about 2.6, the physicalthickness of the capping layer 500 is about 430 Å or more and about 530Å or less. Accordingly, a value range of the desirable physicalthickness of the capping layer 500 may vary, depending on the refractiveindex of the capping layer 500.

A value of the optical thickness represents the optical thickness at awavelength of about 550 nm. That is, the optical thickness is a valueobtained by multiplying the physical thickness by the refractive index,when light of about 550 nm passes through the capping layer 500.

Although not illustrated, a thin film encapsulation layer may be furtherformed on the capping layer 500. The thin film encapsulation layer mayseal and protect the OLED and the driving circuit unit disposed on thesubstrate 110 from the outside.

Hereinafter, the OLED according to an exemplary embodiment of thepresent invention will be described in detail.

FIG. 3 is an enlarged cross-sectional view of the OLED of the OLEDdisplay according to an exemplary embodiment of the present invention.

The OLED of the OLED display according to the present exemplaryembodiment (indicated as X in FIG. 2) has a structure, in which thepixel electrode 191, the hole injection layer 371, the hole transportinglayer 372, the emission layer 373, the electron transporting layer 374,the electron injection layer 375, the common electrode 270, and thecapping layer 500 are sequentially disposed. That is, the organicemission layer 370 of FIG. 2 includes the hole injection layer 371, thehole transporting layer 372, the emission layer 373, the electrontransporting layer 374, and the electron injection layer 375 of FIG. 3.

The hole injection layer 371 may be formed on the pixel electrode 191.In this case, the hole injection layer 371 may be an arbitrary layer forimproving injection of holes from the pixel electrode 191 into the holetransporting layer 372. The hole injection layer 371 may include copperphthalocyanine (CuPc), poly(3,4-ethylenedioxythiophene) (PEDOT),Polyaniline (PANT), N,N-dinaphthyl-N,N′-diphenyl benzidine (NPD), andthe like.

The hole transporting layer 372 may be formed on the hole injectionlayer 371. The hole transporting layer 372 may perform a function ofsmoothly transporting the holes transmitted from the hole injectionlayer 371. For example, the hole transporting layer 372 may includeN,N-dinaphthyl-N,N′-diphenyl benzidine (NPD), TPD(N,N′-bis-(3-methylphenyl)-N,N′-bis-(phenyl)-benzidine), s-TAD,(4,4′,4″-tris (N-3-methylphenyl-N-phenyl-amino)-triphenylamine), and thelike.

In the present exemplary embodiment, the hole transporting layer 372 isdescribed as to be disposed on the hole injection layer 371, thusforming a laminated structure. The hole injection layer 371 and the holetransporting layer 372 may alternatively be formed as a single layer.

Although not illustrated, a buffer layer (not shown) may be furtherdisposed on the hole transporting layer 372. The buffer layer may adjustan amount of holes transferred to the emission layer 373 from the pixelelectrode 191, in addition to an amount of electrons penetrating thehole transporting layer 372 from the emission layer 373. That is, thebuffer layer may control the amount of holes and block the electrons,help to combine the hole and the electrons in the emission layer 373,and block the electrons from penetrating the hole transporting layer372, thereby preventing the hole transporting layer 372 from beingdamaged by the electrons.

The emission layer 373 includes a light-emitting material that exhibitsa specific color. For example, the emission layer 373 may exhibitprimary colors, such as blue, green, and red, or a combination of thesecolors. According to the present exemplary embodiment, the emissionlayer 373 may include a blue emission layer, a green emission layer, anda red emission layer.

The emission layer 373 includes a host and a dopant. The emission layer373 may include a material that emits red, green, blue, and white light,and may be formed using a phosphorescent or fluorescent material.

When emitting red light, the emission layer 373 includes a host materialthat includes CBP (carbazole biphenyl) or mCP (1,3-bis(carbazol-9-yl),and may be made of a phosphorescent material including a dopant, whichincludes at least one of PIQIr(acac)(bis(1-phenylisoquinoline)acetylacetonate iridium), PQIr(acac)(bis(1-phenylquinoline)acetylacetonate iridium), PQIr(tris(1-phenylquinoline)iridium), and PtOEP (octaethylporphyrinplatinum), or alternatively, may be made of a fluorescent materialincluding PBD:Eu(DBM)3(Phen) or perylene.

When emitting green light, the emission layer 373 includes a hostmaterial that includes CBP or mCP, and may be made of a phosphorescentmaterial that includes a dopant material, which includes Ir(ppy)3(fac-tris(2-phenylpyridine)iridium), or alternatively, may be made of afluorescent material including Alq₃ (tris(8-hydroxyquinolino)aluminum).

When emitting blue light, the emission layer 373 includes a hostmaterial including CBP or mCP, and may be made of a phosphorescentmaterial that includes a dopant material, which includes(4,6-F2ppy)2Irpic. Alternatively, the emission layer 373 may be made ofa fluorescent material including at least one of spiro-DPVBi, spiro-6P,distyryl benzene (DSB), distyrylarylene (DSA), a PFO-based polymer, anda PPV-based polymer.

The electron transporting layer 374 may be disposed on the emissionlayer 373. In this case, the electron transporting layer 374 maytransfer electrons from the common electrode 270 to the emission layer373. The electron transporting layer 374 may prevent holes injected fromthe pixel electrode 191 from passing through the emission layer 373 andthen moving to the common electrode 270. That is, the electrontransporting layer 374 serves as a hole blocking layer, and helps tocombine the holes and electrons in the emission layer 373. In this case,the electron transporting layer 374 may include at least one oftris(8-hydroxyquinolino)aluminum (Alq₃), PBD, TAZ, Spiro-PBD, BAlq, andSAlq.

The electron injection layer 375 is disposed on the electrontransporting layer 374. The electron injection layer 375 is an arbitrarylayer that may improve the injection of the electrons from the commonelectrode 270 into the electron transporting layer 374. The electroninjection layer 375 may include Alq₃, LiF, a gallium (Ga) complex, PBD,etc.

Hereinafter, luminous efficiency associated with a thickness of thecapping layer of the OLED display according to an exemplary embodimentof the present invention will be described with reference to FIGS. 4 to7.

FIG. 4 is a graph showing luminous efficiency of white light associatedwith a thickness of a capping layer of the OLED display according to anexemplary embodiment of the present invention. FIG. 5 is a graph showingluminous efficiency of red light associated with the thickness of thecapping layer of the OLED display according to an exemplary embodimentof the present invention. FIG. 6 is a graph showing luminous efficiencyof green light associated with the thickness of the capping layer of theOLED display according to an exemplary embodiment of the presentinvention. FIG. 7 is a graph showing luminous efficiency of blue lightassociated with the thickness of the capping layer of the OLED displayaccording to an exemplary embodiment of the present invention.

As shown in FIG. 4, as the thickness of the capping layer increases fromabout 350 Å to about 830 Å, luminous efficiency of white light graduallyincreases. When the thickness of the capping layer increases at a pointwhere the thickness of the capping layer is about 830 Å, the luminousefficiency of white light gradually decreases.

As shown in FIG. 5, as the thickness of the capping layer increases fromabout 350 Å to about 900 Å, luminous efficiency of red light graduallyincreases.

As shown in FIG. 6, as the thickness of the capping layer increases fromabout 350 Å to about 850 Å, luminous efficiency of green light graduallyincreases.

As shown in FIG. 7, as the thickness of the capping layer increases fromabout 350 Å to about 700 Å, luminous efficiency of blue light graduallyincreases. When the thickness of the capping layer increases from apoint where the thickness of the capping layer is about 700 Å, theluminous efficiency of blue light gradually decreases.

Referring to the graphs illustrated in FIGS. 4 to 7, it can be seen thatthe luminous efficiency is affected as the thickness of the cappinglayer of the OLED display varies. In particular, as the thickness of thecapping layer 500 increases, the luminous efficiency graduallyincreases, and if it exceeds a predetermined thickness, the luminousefficiency gradually decreases. Luminous efficiency of red or greenlight continues to gradually increase as the thickness of the cappinglayer increases. As the thickness of the capping layer increases, theluminous efficiency of blue light increases, but then graduallydecreases once it exceeds a predetermined thickness. Characteristics ofluminous efficiency of white light substantially reflect those of bluelight. This is because the blue pixel represents about half the totalpower consumption of the red, green, and blue pixels. Accordingly, whenpower consumption of the blue pixel is reduced, the entire powerconsumption of the pixels may be reduced, thereby increasing luminousefficiency of white light.

When luminous efficiency of white light is the highest, the thickness ofthe capping layer is about 830 Å. When the thickness of the cappinglayer is about 830 Å, luminous efficiency of blue light is about 100%.Luminous efficiency of blue light is about the similar level, i.e.,100%, when the thickness of the capping layer is about 600 Å. Since theluminous efficiency of the blue pixel is closely associated with theentire luminous efficiency of the pixels, the thickness of the cappinglayer may be about 600 Å or more. When the thickness of the cappinglayer is about 600 Å or less, the luminous efficiency significantlydecreases and thus the power consumption increases.

In FIGS. 4 to 7, the thickness of the capping layer represents aphysical thickness, and the graphs of FIGS. 4 to 7 show experimentalresults when a refractive index thereof is about 1.87. Accordingly, theoptical thickness of the capping layer may be about 600 Å*1.87 or more,i.e., about 1100 Å or more.

Hereinafter, an amount of color shift associated with the thickness ofthe capping layer of the OLED display according to an exemplaryembodiment of the present invention will be described with reference toFIGS. 8 to 11. As used herein, the amount of color shift may be avariation in color that is associated with a viewing angle, whichrepresents a difference in color between a front view and a side view.As the amount of color shift increases, the difference in colorassociated with the viewing angle increases. When the amount of colorshift is reduced, the viewing angle may be improved. The amount of colorshift is digitized as u′v′.

FIG. 8 is a graph showing an amount of color shift for white associatedwith the thickness of the capping layer of the OLED display according toan exemplary embodiment of the present invention. FIG. 9 is a graphshowing an amount of color shift for red associated with the thicknessof the capping layer of the OLED display according to an exemplaryembodiment of the present invention. FIG. 10 is a graph showing anamount of color shift for green associated with the thickness of thecapping layer of the OLED display according to an exemplary embodimentof the present invention. FIG. 11 is a graph showing an amount of colorshift for blue associated with the thickness of the capping layer of theOLED display according to an exemplary embodiment of the presentinvention.

As shown in FIG. 8, when the thickness of the capping layer is about 830Å to about 730 Å, an amount of color shift for white shows similarlevels, and when the thickness of the capping layer is about 730 Å orless, the amount of color shift for white gradually decreases as thethickness of the capping layer decreases. When the thickness of thecapping layer decreases from about 830 Å to about 780 Å, the amount ofcolor shift for white slightly increases, and when the thickness of thecapping layer is about 730 Å, the amount of color shift for white showsa level similar to that of about 830 Å.

As shown in FIG. 9, when the thickness of the capping layer decreasesfrom about 830 Å to about 680 Å, an amount of color shift for reddecreases from about 0.058 to about 0.039.

As shown in FIG. 10, when the thickness of the capping layer decreasesfrom about 830 Å to about 680 Å, an amount of color shift for greendecreases from about 0.023 to about 0.011.

As shown in FIG. 11, when the thickness of the capping layer decreasesfrom about 830 Å to about 680 Å, an amount of color shift for bluedecreases from about 0.037 to about 0.027.

Referring to the graphs illustrated in FIGS. 8 to 11, it can be seenthat the amount of color shift is affected as the thickness of thecapping layer of the OLED display varies. Generally, as the thickness ofthe capping layer decreases, the amount of color shift graduallydecreases. The amount of color shift for white represents the amount ofcolor shift associated with the various thicknesses of the cappinglayer, and when the thickness of the capping layer decreases for thered, green, and blue colors, the amounts of color shift are respectivelyreduced.

When the thickness of the capping layer is about 830 Å, the amount ofcolor shift for white is about 0.022. The amount of color shift forwhite is about similar level, i.e., 0.022, when the thickness of thecapping layer is about 730 Å. In order to have a lower amount of colorshift than when the thickness of the capping layer is about 830 Å, thethickness of the capping layer may be about 730 Å or less.

In FIGS. 8 to 11, the thickness of the capping layer represents aphysical thickness and the graphs of FIGS. 8 to 11 show experimentalresults when a refractive index thereof is about 1.87. Accordingly, anoptical thickness of the capping layer may be about 730 Å*1.87 or less,i.e., about 1400 Å or less.

Through the analysis of the graphs of FIGS. 4 to 11, the opticalthickness of the capping layer may be about 600 Å*1.87 or more and about730 Å*1.87 or less. That is, the optical thickness of the capping layermay be about 1100 Å or more and about 1400 Å or less, to improveluminous efficiency and minimize the amount of color shift associatedwith the viewing angle.

Hereinafter, the OLED display according to an exemplary embodiment ofthe present invention will be described with reference to FIG. 12.

The OLED display according to the present exemplary embodimentillustrated in FIG. 12 has substantially the same configuration as theOLED display illustrated with reference to FIGS. 1 to 3, and, thus,repeated description thereof will be omitted. The present exemplaryembodiment differs from the aforementioned OLED display in that anemission layer includes a layer including only a host, which will bedescribed below in detail.

FIG. 12 is a cross-sectional view of an organic emission layer of theOLED display according to an exemplary embodiment of the presentinvention.

As shown in FIG. 12, the organic emission layer 370 of the OLED displayaccording to the present exemplary embodiment includes the holeinjection layer 371, the hole transporting layer 372, the emission layer373, the electron transporting layer 374, and the electron injectionlayer 375. The hole injection layer 371, the hole transporting layer372, the emission layer 373, the electron transporting layer 374, andthe electron injection layer 375 may be sequentially disposed. It isnoted that, however, a portion the layers above constituting the organicemission layer 370 may be omitted.

The emission layer 373 includes a first emission layer 373 a and asecond emission layer 373 b. The first emission layer 373 a includes ahost, but not a dopant, and, thus, may be referred to as a non-dopinglayer. The second emission layer 373 b includes a host and a dopant,and, thus, may be referred to as a doping layer. The first emissionlayer 373 a contacts the hole transporting layer 372, and the secondemission layer 373 b contacts the electron transporting layer 374. It isnoted that, however, the first emission layer 373 a may alternativelycontact the hole injection layer 371 and the second emission layer 373 bmay contact the electron injection layer 375. In this case, the holetransporting layer 372 or the electron transporting layer 374 may beomitted.

The second emission layer 373 b is disposed in center and upper regionsof the emission layer 373. A thickness ratio of the first emission layer373 a to the second emission layer 373 b may be about 1:3. It is notedthat, however, the thickness ratio of the first emission layer 373 a tothe second emission layer 373 b may widely vary.

In the present exemplary embodiment, a driving voltage may be reduced,since the emission layer 373 includes the non-doping layer that includesthe host but not the dopant, thereby increasing the luminous efficiencyand reducing the color shift associated with the viewing angle.

The OLED display according to the present exemplary embodiment mayinclude pixels. In this case, the pixels may include first color pixelsPX1, second color pixels PX2, and third color pixels PX3, as shown inFIG. 24. For example, the first color pixel PX1 may be a green pixel,the second color pixel PX2 may be a red pixel, and the third color pixelPX3 may be a blue pixel. In this case, only an emission layer 373 of thefirst color pixel PX1 may include the first emission layer 373 a and thesecond emission layer 373 b, and emission layers 373 of the second PX2and third color pixels PX3 may include only the second emission layer373 b.

That is, only the green pixel may have a structure in which the emissionlayer 373 is partially doped. This is because the red and blue pixelsmay not be affected by the reduced driving voltage as the green pixel.It is noted that, however, all of the first color pixel, second colorpixel, and third color pixel may alternatively have a structure in whichthe emission layer 373 is partially doped. In the above description, thefirst emission layer 373 a and the second emission layer 373 b areinterchangeable with each other.

Hereinafter, an organic emission layer of an OLED display according toan exemplary embodiment of the present invention will be described withreference to FIG. 13.

FIG. 13 is a cross-sectional view of an organic emission layer of theOLED display according to an exemplary embodiment of the presentinvention.

Referring to FIG. 13, the organic emission layer 370 of the OLED displayaccording to the present exemplary embodiment includes a hole injectionlayer 371, a hole transporting layer 372, an emission layer 373, anelectron transporting layer 374, and an electron injection layer 375.

The emission layer 373 includes a first emission layer 373 a including ahost, but not a dopant, and a second emission layer 373 b including ahost and a dopant. The first emission layer 373 a contacts the holetransporting layer 372, and also contacts the electron transportinglayer 374. It is noted that, however, the first emission layer 373 a mayalternatively contact the hole injection layer 371, and may also contactthe electron injection layer 375. In this case, the hole transportinglayer 372 or the electron transporting layer 374 may be omitted.

The second emission layer 373 b is disposed in a center region of theemission layer 373. The first emission layers 373 a are disposed aboveand below the second emission layer 373 b. A thickness ratio of thefirst emission layer 373 a to the second emission layer 373 b may beabout 1:1. It is noted that, however, the thickness ratio of the firstemission layer 373 a to is the second emission layer 373 b may widelyvary.

For comparison with the present exemplary embodiment, a structure of anOLED display according to a comparative embodiments will be describedwith reference to FIGS. 14 and 15.

FIGS. 14 and 15 are cross-sectional views of an organic emission layerof an OLED display according to comparative embodiments.

Referring to FIG. 14, the emission layer 373 includes a first emissionlayer 373 a including a host, but not a dopant, and a second emissionlayer 373 b including a host and a dopant. The first emission layer 373a contacts an electron transporting layer 374, and the second emissionlayer 373 b contacts a hole transporting layer 372. The second emissionlayer 373 b is disposed in center and lower regions of the emissionlayer 373. A thickness ratio of the first emission layer 373 a to thesecond emission layer 373 b may be about 1:3.

Referring to FIG. 15, the emission layer 373 includes the first emissionlayer 373 a that includes a host, but not a dopant, and the secondemission layer 373 b that includes a host and a dopant. The secondemission layer 373 b contacts the hole transporting layer 372, and alsocontacts the electron transporting layer 374. The first emission layer373 a is disposed in a center region of the emission layer 373. Thesecond emission layers 373 b are disposed above and below the firstemission layer 373 a. A thickness ratio of the first emission layer 373a to the second emission layer 373 b may be about 1:1.

Referring to FIG. 16, an amount of color shift of the OLED displayaccording to an exemplary embodiment of the present invention will bedescribed.

FIG. 16 is a graph showing an amount of color shift associated with astructure of the emission layer of the OLED display according to anexemplary embodiment of the present invention. FIG. 16 shows an amountof color shift for green associated with a structure of an emissionlayer of a green pixel.

In a structure where the emission layer is entirely doped by a dopinglayer that includes a host and a dopant, the amount of color shiftaccording to the viewing angle is about 0.013. In a structure where theemission layer is partially doped by a non-doping layer that includes ahost, but not a dopant, the amount of color shift associated with theviewing angle is about 0.011. It can be seen that in the structure wherethe emission layer is partially doped, the amount of color shiftassociated with the viewing angle improves by about 16%.

In addition, when the optical thickness of the capping layer is about1122 Å (600 A*1.87) or more and about 1365.1 Å (730 Å*1.87) or less, andthe emission layer has the structure where the emission layer ispartially doped, the amount of color shift associated with the viewingangle may be further improved.

Hereinafter, a driving voltage and luminous efficiency of the OLEDdisplay according to an exemplary embodiment of the present inventionwill be described with reference to FIGS. 17 and 18.

FIG. 17 is a graph showing a driving voltage associated with thestructure of the emission layer of the OLED display according to anexemplary embodiment of the present invention. FIG. 18 is a graphshowing luminous efficiency associated with the structure of theemission layer of the OLED display according to an exemplary embodimentof the present invention.

Referring to FIG. 17, in the structure where the emission layer isentirely doped by a doping layer including a host and a dopant (Ref.), adriving voltage thereof is about 6.5V. A driving voltage of the OLEDdisplay according to the exemplary embodiment of present inventionillustrated with reference to FIG. 12 is about 5.7V (case 1). That is,in the structure where the first emission layer contacts the holetransporting layer, the second emission layer contacts the electrontransporting layer, and the thickness ratio of the first emission layerto the second emission layer is about 1:3, a driving voltage isdecreased further than in the structure where the emission layer isentirely doped (Ref.). A driving voltage of the OLED display accordingto the exemplary embodiment of the present invention illustrated withreference to FIG. 13 is about 5.5V (case 2). That is, in the structurewhere the first emission layer contacts the hole transporting layer andthe electron transporting layer, the second emission layer is disposedbetween the first emission layers, and the thickness ratio of the firstemission layer to the second emission layer is about 1:1, a drivingvoltage is decreased further than in the structure where the emissionlayer is entirely doped (Ref.).

A driving voltage of the OLED display according to the comparativeembodiment illustrated with reference to FIG. 13 is about 6.2V (case 3).That is, in the structure where the first emission layer contacts theelectron transporting layer, the second emission layer contacts the holetransporting layer, and a thickness ratio of the first emission layer tothe second emission layer is about 1:3, a driving voltage is decreasedfurther than in the structure where the emission layer is entirely doped(Ref), but the difference between the driving voltages is notsignificant. A driving voltage of the OLED display according to thecomparative embodiment illustrated with reference to FIG. 15 is about6.9V (case 4). That is, in the structure where the second emission layercontacts the hole transporting layer and the electron transportinglayer, the first emission layer is disposed between the second emissionlayers, and the thickness ratio of the first emission layer to thesecond emission layer is about 1:1, a driving voltage is increasedfurther than in the structure where the emission layer is entirely doped(Ref.).

Referring to FIG. 18, in the structure where the emission layer isentirely doped (Ref.) by including a doping layer that includes a hostand a dopant, luminous efficiency is about 113%. In case 1 illustratedwith reference to FIG. 12, luminous efficiency thereof is about 122%.That is, in the structure where the first emission layer contacts thehole transporting layer, the second emission layer contacts the electrontransporting layer, and the thickness ratio of the first emission layerand the second emission layer is about 1:3, the luminous efficiencyincreases further than in the structure where the emission layer isentirely doped (Ref.). In case 2 illustrated with reference to FIG. 13,luminous efficiency thereof is about 125%. That is, in the structurewhere the first emission layer contacts the hole transporting layer andthe electron transporting layer, the second emission layer is disposedbetween the first emission layers, and the thickness ratio of the firstemission layer and the second emission layer is about 1:1, luminousefficiency increases further than in the structure where the emissionlayer is entirely doped (Ref.).

In case 3, illustrated with reference to FIG. 14, luminous efficiencythereof is about 115%. That is, in the structure where the firstemission layer contacts the electron transporting layer, the secondemission layer contacts the hole transporting layer, and the thicknessratio of the first emission layer to the second emission layer about1:3, the luminous efficiency increases further than in the structurewhere the emission layer is entirely doped (Ref), but the luminousefficiency is slightly different therefrom. In case 4, illustrated withreference to FIG. 15, luminous efficiency thereof is about 96%. That is,in the structure where the second emission layer contacts the holetransporting layer and the electron transporting layer, the firstemission layer is disposed between the second emission layers, and thethickness ratio of the first emission layer to the second emission layeris about 1:1, the luminous efficiency decreases further than in thestructure where the emission layer is entirely doped (Ref.).

Referring to FIGS. 17 and 18, where the emission layer is partiallydoped, the second emission layer is disposed in the center region of theemission layer, and the first emission layer contacts the hole injectionlayer or the hole transporting layer, according to the present exemplaryembodiment, the driving voltage thereof may be decreased and luminousefficiency thereof may be increased further than in the structure wherethe emission layer is entirely doped. Even if the emission layer has thestructure where the emission layer is partially doped (as in the Ref.),an effect of the reduced driving voltage may not be significant, or thedriving voltage may be increased in the structure where the secondemission layer contacts the hole injection layer or the holetransporting layer.

As described above, when the optical thickness of the capping layer isabout 1122 Å (600 Å*1.87) or more and about 1365.1 Å (730 Å*1.87) orless, the emission layer is partially doped, the second emission layeris disposed in the center region of the emission layer, and the firstemission layer contacts the hole injection layer or the holetransporting layer, the driving voltage thereof may be further decreasedand the luminous efficiency can be further increased.

Hereinafter, an OLED display according to an exemplary embodiment of thepresent invention will be described with reference to FIG. 19.

Referring to FIG. 19, the OLED display according to the presentexemplary embodiment has substantially the same configuration as theOLED display illustrated with reference to FIGS. 1 to 3, and, thus,detailed description thereof will be omitted. The present exemplaryembodiment differs from the aforementioned OLED display in that acapping layer includes multiple layers, which will be described below indetail.

FIG. 19 is a cross-sectional view of an OLED of the OLED displayaccording to an exemplary embodiment of the present invention.

The OLED according to the present exemplary embodiment has a structure,in which a pixel electrode 191, a hole injection layer 371, a holetransporting layer 372, an emission layer 373, an electron transportinglayer 374, an electron injection layer 375, a common electrode 270, anda capping layer 500 are sequentially disposed.

The capping layer 500 includes first and second capping layers 510 and520 that have different refractive indices from each other. The secondcapping layer 520 is disposed on the first capping layer 510. Arefractive index of the second capping layer 520 is higher than arefractive index of the first capping layer 510. Accordingly, the firstand second capping layers 510 and 520 include different materials. Adifference between the refractive index of the first capping layer 510and the refractive index of the second capping layer 520 is about 0.1 ormore and about 1.4 or less. The refractive index of the first cappinglayer 510 may be about 1.2 or more and about 1.5 or less. The refractiveindex of the second capping layer 520 may be about 1.6 or more and about2.6 or less. For example, the refractive index of the first cappinglayer 510 may be about 1.38, and the refractive index of the secondcapping layer 520 may be about 2.05.

An optical thickness of the first capping layer 510 may be about 60 Å ormore and about 210 Å or less. When the refractive index of the firstcapping layer 510 is about 1.38, a physical thickness of the firstcapping layer 510 may be about 50 Å or more and about 150 Å or less.That is, the optical thickness of the first capping layer 510 may beabout 50 Å*1.38 or more and about 150 Å*1.38 or less. A value range ofthe desirable physical thickness of the first capping layer 510 may varydepending on the refractive index of the first capping layer 510.

An optical thickness of the second capping layer 520 may be about 820 Åor more and about 1030 Å or less. When the refractive index of thesecond capping layer 520 is 2.05 or more, a physical thickness of thesecond capping layer 520 may be about 400 Å or more and about 500 Å orless. That is, the optical thickness of the second capping layer 520 maybe about 400 Å*2.05 or more and about 500 Å*2.05 or less. A value rangeof the desirable physical thickness of the second capping layer 520 mayvary depending on the refractive index of the second capping layer 520.As used herein, a value of the optical thickness may represent theoptical thickness at a wavelength of about 550 nm. That is, the opticalthickness is a value that is obtained by multiplying the physicalthickness with the refractive index when light of about 550 nm passesthrough the capping layer 500.

Hereinafter, luminous efficiency associated with thicknesses of firstand second capping layers of the OLED according to an exemplaryembodiment of the present invention will be described with reference toFIGS. 20 and 21.

FIG. 20 is a graph showing luminous efficiency associated with athickness of a first capping layer of the OLED display according to anexemplary embodiment of the present invention. FIG. 21 is a graphshowing luminous efficiency associated with a thickness of a secondcapping layer of the OLED display according to an exemplary embodimentof the present invention.

Referring to FIG. 20, as a first thickness of the first capping layerincreases from about 50 Å to about 150 Å, luminous efficiency graduallyincreases. When the first thickness of the first capping layer increasesfrom a point where the first thickness of the first capping layer isabout 150 Å, the luminous efficiency gradually decreases.

Referring to FIG. 21, as a second thickness of the second capping layerincreases from about 400 Å to about 500 Å, the luminous efficiencygradually increases. When the second thickness of the second cappinglayer increases from a point where the second thickness of the secondcapping layer is about 500 Å, the luminous efficiency graduallydecreases.

Hereinafter, an amount of color shift associated with the thicknesses ofthe first and second capping layers of the OLED display according to anexemplary embodiment of the present invention will be described withreference to FIGS. 22 and 23.

FIG. 22 is a graph showing an amount of color shift associated with thethickness of the first capping layer of the OLED display according to anexemplary embodiment of the present invention. FIG. 23 is a graphshowing amount of color shift associated with the thickness of thesecond capping layer of the OLED display according to an exemplaryembodiment of the present invention.

Referring to FIG. 22, as the first thickness of the first capping layerincreases from about 50 Å to about 150 Å, an amount of color shiftgradually increases. When the first thickness of the first capping layeris between about 150 Å and about 200 Å, the amount of color shift doesnot vary, but the amount of color shift gradually decreases when thefirst thickness of the first capping layer increases from a point wherethe first thickness is about 200 Å.

Referring to FIG. 23, as the second thickness of the second cappinglayer increases from about 400 Å to about 550 Å, the amount of colorshift gradually increases. When the second thickness of the secondcapping layer increases from a point where the second thickness of thesecond capping layer is about 550 Å, the amount of color shift graduallydecreases.

In consideration of data from FIGS. 20 to 23, a desirable thicknessrange of the first and second capping layers may be obtained. Whenconsidering only the luminous efficiency, the first thickness of thecapping layer may be about 50 Å to about 200 Å, and the second thicknessof the capping layer may be about 400 Å to about 600 Å. In the presentexemplary embodiment, both the amount of color shift associated with theviewing angle and the luminous efficiency are taken into account.Accordingly, the first thickness of the capping layer may be about 50 Åand about 150 Å, and the second thickness of the capping layer may beabout 400 Å and about 500 Å.

The thicknesses of the first and second capping layers in FIGS. 20 to 23represent the physical thicknesses, and the graphs of FIGS. 20 to 23show experimental results when a refractive index of the first cappinglayer is 1.38 and a refractive index of the second capping layer is2.05. Accordingly, an optical thickness of the first capping layer maybe about 50 A*1.38 or more and about 150 Å*1.38 or less, and an opticalthickness of the second capping layer may be about 400 Å*2.05 or moreand about 500 Å*2.05 or less.

In addition, the optical thickness of the first capping layer may beabout 60 Å or more and about 210 Å or less, and the optical thickness ofthe second capping layer may be about 820 Å or more and about 1030 Å orless, the emission layer may have the structure where the emission layeris partially doped, the second emission layer may be disposed in thecenter region of the emission layer, and the first emission layer cancontact the hole injection layer or the hole transporting layer, therebyfurther increasing the luminous efficiency and further improving theamount of color shift associated with the viewing angle.

Although certain exemplary embodiments and implementations have beendescribed herein, other embodiments and modifications will be apparentfrom this description. Accordingly, the inventive concept is not limitedto such exemplary embodiments, but rather to the broader scope of thepresented claims and various obvious modifications and equivalentarrangements.

What is claimed is:
 1. An organic light emitting diode (OLED) display,comprising: a first color pixel, a second color pixel, and a third colorpixel on a substrate, each of the first, second, and third color pixelscomprising: a first electrode disposed on the substrate; an organicemission layer disposed on the first electrode; a second electrodedisposed on the organic emission layer; and a capping layer disposed onthe second electrode, wherein: the first color pixel is configured toemit green light, and the second and third color pixels are eachconfigured to emit a color of light other than green; the organicemission layer of the first color pixel comprises a first emission layerand a second emission layer each being configured to emit light; theorganic emission layer of the second color pixel or the third colorpixel comprises a third emission layer configured to emit light; thesecond emission layer and the third emission layer comprise both a hostand a dopant; and the first emission layer comprises the host, and doesnot comprise any dopants therein.
 2. The OLED display of claim 1,wherein a refractive index of the capping layer is in a range of about1.6 to about 2.6.
 3. The OLED display of claim 2, wherein a physicalthickness of the capping layer is in a range of about 600 Å to about 730Å when the refractive index of the capping layer is about 1.87.
 4. TheOLED display of claim 2, wherein the refractive index of the cappinglayer is measured at a wavelength of about 550 nm.
 5. The OLED displayof claim 1, wherein the capping layer comprises an organic material oran inorganic material.
 6. The OLED display of claim 5, wherein thecapping layer comprises at least one of a triamine derivative, anarylenediamine derivative, CBP, and tris(8-hydroxyquinoline) aluminum(Alq3).
 7. The OLED display of claim 1, wherein an optical thickness ofthe capping layer is in a range of about 1100 Å to about 1400 Å.
 8. TheOLED display of claim 1, wherein the organic emission layer of the firstcolor pixel further comprises: a hole injection layer and a holetransporting layer disposed on the first electrode; and an electrontransporting layer and an electron injection layer.
 9. The OLED displayof claim 8, wherein: the first emission layer contacts the holeinjection layer or the hole transporting layer; and the second emissionlayer contacts the electron transporting layer or the electron injectionlayer.
 10. The OLED display of claim 9, wherein: the organic emissionlayer of the first color pixel further comprises another first emissionlayer; and the second emission layer is disposed between the firstemission layer and the another first emission layer.
 11. The OLEDdisplay of claim 10, wherein the second emission layer is disposed in acenter region of the organic emission layer of the first color pixel.12. The OLED display of claim 10, wherein: the first emission layerdisposed on the second emission layer contacts the hole injection layeror the hole transporting layer; and the another first emission layerdisposed under the second emission layer contacts the electrontransporting layer or the electron injection layer.
 13. The OLED displayof claim 1, wherein: the organic emission layer of the first color pixelfurther comprises another first emission layer; and the second emissionlayer is disposed between the first emission layer and the another firstemission layer.